1. Field of the Invention
This invention pertains to the field of elastomers having a polyurethane binder. More particularly, it pertains to such elastomers formed from a polyol mixture having a high proportion of plasticizer, the elastomers being useful for voidless films, energetic compositions, and inert simulants for energetic compositions.
2. Description of the Prior Art
Elastomeric materials which have high tensile strength and great elongation to failure are desired for a variety of purposes. It is highly desirable that such elastomeric materials retain these properties although including fillers and adventious materials that are not elastomeric. These properties are enhanced by the absence of voids, and voidless elastomeric materials are required for leak-proof articles formed of films of such materials. Elastomers generated by curing from a liquid are particularly desirable since they may be formed by a variety of processes and in many shapes.
Elastomeric characteristics of materials related to the present invention are provided by the bending, rotation, and uncoiling of cross-linked polymeric chains or backbones forming a binder which determines the ultimate properties of the materials. In a relaxed state of the materials, flexible segments of the chains form coils which uncoil upon extension until rupture occurs. The length of the coiled segments between cross-links or rigid chain portions such as urethanes thus limits the extension that can be achieved prior to rupture.
However for elongation to occur, the chains must be separated by non-volatile liquid plasticizers which allow portions of the chains to move relatively. As a result, the toughness of an elastomer, which for the purposes of the present application is represented by the area under the stress/strain curve of the elastomer, is mainly determined by interaction between the binder and plasticizer, although dependent on other factors such as the kind and amount of curative and interactions of the binder and plasticizer with any relatively rigid fillers, for example, the particulate materials typically used to provide additional energy in energetic elastomeric materials. Toughness contributes to the insensitivity of energetic elastomers materials to undesired initiation and is, of course, highly desirable in other elastomeric articles such as coatings, flexible and expansible containers, caulking compounds, potting materials, adhesives, and articles formed by films or membranes.
Many binder and plasticizer materials are known; and U.S. Pat. No. 4,799,980 to Reed, Jr., one of the present inventors, which issued 24 Jan. 1989 and which is incorporated herein by reference, describes a variety of elastomers formed of these materials and useful for energetic materials, that is, explosives and propellants. Such energetic materials typically include particulate organic fillers and metals and yet must retain their elastomeric properties. However, elastomeric materials using the same or similar binders are useful for inert simulants for the energetic elastomers and are useful for elastomeric articles for any purposes wherein high tensile strength, great elongation, and toughness are desirable.
A particular problem with elastomeric materials is the presence of voids and microporosities which reduce strength and elongation and whose absence is essential for protective coatings and for articles such as containers, including bladders for supplying fluids, and for articles, such as condoms and gloves, for preventing the transmission of viruses and other substances injurious to health. Elastomers formed by evaporation of a liquid, which may be a solvent or may be a carrier liquid as in latexes, tend to develop voids. Voids also develop where polymerization or coagulation of particles is interrupted by solid impurities that are not wetted by the curing elastomer, as is the case with the macromolecules forming latexes.
It is thus highly desirable to provide elastomers formed without volatile solvents and from liquids composed of relatively small molecules that are able to penetrate into the surface microstructure of solids. Voids are also avoided by elastomers formed by polymerization, so that there is essentially no shrinkage, and by polyols having sufficient molecular weight to minimize exothermic effects. For avoidance of voids it is desirable that an elastomer have polar groups that enhance bonding to many substances although without forming bonds rigid enough to reduce elongation. Such elastomers formed without volatile solvents and by polymerization of microstructure penetrating molecules without shrinkage and with polar bonding, as to substrates, without forming bonds rigid enough to reduce elongation are also elastomers most effective for coating, caulking, potting, and adhesive purposes.
A desirable feature of elastomers used for certain purposes, such as bladders, is that the polymer chains of the elastomer be uncoiled easily so stress increases slowly with increasing strain. This provides a relatively constant force over a wide range of elongation. However, continued elongation brings the chains closer together so that interaction between cross-links or chains occurs and provides high ultimate strength.
For processing, as by casting or extrusion, it is required that an elastomeric material have suitable rheological properties before curing and even though including particulate fillers. Also for processing, it is highly desirable that an elastomeric system having a variety of uses be adapted to vary the rate of curing and that curing be possible over a range of easily provided temperatures. It is further desirable that polymerization be possible in situ at the microsurfaces of solid fillers and impurities for adhesion of the binder thereto.
It is also desirable that an elastomers be adapted for the inclusion of solid ingredients as medicinals or antiseptics by dissolving them in the liquid binder solution.
various aspects of elastomers related to the present invention will now be discussed in greater detail. One aspect is that the functionality of polyols used to form elastomeric compositions has a dominant effect on the area under the stress-strain curve of the elastomer. For example, compositions having a difunctional polyethylene glycol (PEG) binder display a slowly rising area as chain length, measured by backbone atoms, increases. In such a difunctional PEG there are two carbons and one oxygen, or three skeletal atoms per mer unit. As these units are increased, elongation at rupture increases modestly while stress decreases because of the continuing decrease in cross-link density. Plasticized compositions containing a difunctional PEG having a molecular weight of 4500 daltons, such as those widely used in modern high energy, high elongation propellants, tend to have values of tensile stress at failure and elongations that are one-half or less than those of corresponding tetrafunctional polyalkalyene oxide (PAO) compositions. Such properties are the result of incomplete cross-linking and a partially formed polymer network; and it has been reported that cross-link density of PEG/nitroglycerine (NG) elastomers decrease markedly as a plasticizer/binder ratio by weight (Pl/Po) of 3 is exceeded.
However, compositions containing polyols with functionality of three or four are much tougher and exhibit a greater dependence on skeletal atoms. In these compositions, all hydroxyl groups should have equal reactivity so that a regular network can be formed, and the hydroxyls should be unhindered and primary since secondary hydroxyl groups react more slowly and, perhaps, less completely, than primary and thus limit the molecular weight of the network. Similarly, it is desirable that the urethane forming groups of a curative have equal reactivity.
At the higher functionalities, a given chain length is more effective because the quaternary and tertiary carbon cross-links are more efficient than urethane cross-links which may be relatively fragile, since they are polar and rigid in nature, as compared to the resilient carbon cross-links in the polyol moiety. As a result, carbon cross-links are critical in attaining regular and fully cross-linked polymer networks. As a polyether elastomer having such networks is extended, the polyether is uncoiled easily so that stress increases slowly as strain increases with continued elongation bringing the chains closer together so that interaction between urethane cross-links or chains occurs and the stress rises increasingly with further strain.
Films and adherent coatings formed from hydroxyl terminated polyethers provide liquid curable, plasticized urethanes which are single-phase and contain no volatile solvents or other substances to create voids and for which nonvolatile polyols capped with isocyanates are effective as relatively non-toxic plasticizers. As before mentioned, liquids that can be polymerized to form elastomers have an inherent ability to bond to fillers and to substrates since these liquids are composed of relatively small molecules that are able to penetrate into the surface microstructure of solids to a degree not possible for the macromolecules contained in many lattices and since in situ polymerization of polyols occurring in the microsurface tends to enhance adhesion between the polymer-filler adhesion. Voids are also avoided by binders formed from relatively high molecular weight polyols, such as those having a weight of 15,000 or more daltons, which have essentially no shrinkage as polymerization occurs. Polyols of such weights also minimize polymerization exothermic effects that might damage binders or fillers.
Polyether binders have an affinity for polar solids forming substrates, intentional fillers, and adventious impurities. While solids that have a strong affinity for binders are generally not desirable because they reduce elongation, as by preventing the formation of coiled flexible segments; binders and solids with no affinity undergo early and irreversible dewetting, the tendency of a binder to pull away from such solids when the composite of binder, plasticizer, and composite undergoes tensile stress. However, the weak affinity of hard urethane linkages for such solids desirably reduces dewetting without adversely reducing elongation.
This advantage of urethane groups is also provided by the amide linkage in proteins, which bonds tenaciously to polar substrates of polar materials. However, the amide units are not separated by long flexible segments so that the proteins have little elasticity and synthetic amides are not conveniently formed by the curing of liquids at ambient temperature.
Polyols, such as the oxetanes, having large pendant groups tend to have reduced toughness, perhaps because of internal plasticization contributed by the pendant group which, in any event, reduces the number of load-bearing atoms between cross-links.
Despite the advantages of liquid polyethers, some such as polypropylene glycol (PPG), polytetramethylene glycol (PTMG), and butylene glycol (BG), are incompatible with inert and energetic plasticizers. PPG and BG also have secondary hydroxyl groups which react more slowly and less completely than primary hydroxyls. It is thus more difficult to attain toughness with these glycols than with other liquid polyalkylene glycols. Even with binders which are predominately PEG, some plasticizers such as the nitrate esters used in energetic elastomers tend to inhibit cross-linking and thus limit molecular weight after polymerization, toughness, and plasticizer retention.
From the above it is believed apparent that elastomeric compositions having a polyurethane binder formed from a curable liquid which includes, together with a suitable plasticizer and a suitable curative, a tetrafunctional polyalkalyene oxide polyol having a relatively high molecular weight would have favorable tensile strength, elongation, bonding with fillers and impurities, and absence of voids. However, heretofore those skilled in the art have limited the plasticizer/binder ratio of such compositions to about 3.0 since, in related prior art compositions, the excess plasticizer exudes making these compositions dangerous, as with nitrate ester plasticizers, impractical as with health care articles and containers and, at least, undesirable for many purposes as where long storage and high ambient temperatures are involved. In the prior art it was also believed that, as pointed out above in connection with of polyethylene glycol/nitroglycerine elastomers, such a high Pl/Po decreases cross-link density resulting in lower tensile strength or elongation without any improvement in toughness and flexibility without breakage. As a result and when use of such a tetrafunctional polyalkalyene oxide polyol was contemplated in the prior art--see for example composition "DRX-4" of Table 2 of in the above U.S. Pat. No. 4,799,980 to one of the present inventors wherein the Pl/Po ratio was limited to 3.16 although the invention was directed to tough energetic compositions--relatively no more plasticizer was used than previously employed with difunctional polyalkalyene oxide derived compositions.
Explosive compositions which have high performance, toughness, and flexibility and which can be loaded into relatively long lengths of plastic tubing are believed unreported in the prior art. Curable such explosive compositions having elastomeric binders lack toughness and flexibility and thus break when bent. Also, the curable compositions cannot be loaded into such lengths of tubing because they contain high levels (.gtoreq.280%) of relatively coarse solids (.gtoreq.2150 m particle diameter). Such explosive compositions have been provided in the form of noncurable pastes; but these pastes are unsatisfactory because they separate when loaded into tubes stored in a folded configuration. In these prior art compositions, breakage and separation adversely affects detonation propagation; and the performance of these compositions is limited because their limited flexibility requires a relatively high weight of inert tubing.
For development of and training in the use of a system using such a net and for evaluation of the properties of any explosive used therein--as well as for any other elastomeric explosive and propellant compositions, it is highly desirable that inert simulants be provided having substantially the same physical and mechanical properties as such energetic compositions.